1. Field of the Invention
Aspects of the present invention relate to a holographic data storage device, and more particularly, to an apparatus for recording/reproducing holographic data and a method of adjusting a position of a recording layer, by which an initial position of the recording layer and a position of a desired recording layer can be effectively adjusted during recording.
2. Description of the Related Art
Technology for storing information using holograms is widely used. In such technology, information is stored in an optical interference pattern in an inorganic crystal that is sensitive to light or in a material that reacts according to intensity of light, such as a photo polymer. Optical interference patterns are formed using two coherent laser beams. In other words, interference patterns are formed when a reference light and a signal light having different paths interfere with each other, thereby causing chemical or physical changes in a photosensitive information storage medium and are recorded thereon. A reference light similar to light used to record information is irradiated on interference patterns recorded on the information storage medium so as to reproduce information from the recorded interference patterns. This causes diffraction due to the interference patterns. The signal light is restored and information is reproduced.
Examples of technology for storing information using holograms include a volume holography method by which information is recorded/reproduced in one page units using volume holography, and a micro-holography method by which information is recorded/reproduced in single bit units by using micro-holography. In the volume holography method, a large amount of information can be processed simultaneously. However, since an optical system should be very precisely adjusted, it is not easy to adopt this method in consumer devices.
In the micro-holography method, two condensed light beams interfere with each other at a focal point and thus form fine interference patterns (micro-hologram). Positions on which the interference patterns are formed are moved on a plane of an information storage medium, so that a plurality of interference patterns are recorded on the information storage medium to form a plurality of recording layers. The recording layers are superimposed in a depth direction of the information storage medium to form a multi-layer structure so that information can be recorded three-dimensionally on the information storage medium.
In the micro-holography method, information is recorded in a plurality of recording layers in the depth direction of the information storage medium, thereby increasing a recording capacity of the information storage medium. In a multi-layer optical disc, such as a Blu-ray disc (BD), a reflective layer exists in each of the recording layers, and the plurality of recording layers are discriminated according to a level of the intensity signal of reflected light and the polarity of the signal, and an optical focus is formed in a desired recording layer.
However, in the information storage medium used in the micro-holography method, a reflective layer does not exist in each of the recording layers unlike in existing optical discs. When a reflective layer exists in each of the recording layers in the holographic information storage medium, efficiency of recorded light is degraded due to the reflective layer. Thus, a plurality of layers cannot be recorded in the depth direction of the information storage medium, and a recording capacity may not be increased.
Thus, in the information storage medium used in the micro-holography method, a reflective layer does not exist in each of the recording layers. As such, it is not easy to form an optical focus in a desired recording layer of the information storage medium. When an optical focus is not formed on the desired recording layer, a distance between the recording layers may not be uniform. As a result, crosstalk between adjacent layers may not be uniform, and the performance of a reproduction signal in each of the recording layers may be different. Furthermore, the optical focus may be formed in a portion that is far away from the recording layers, and recording may not be appropriately performed even when recording starts.